An organic solvent type coating composition has the problems of atmospheric pollution with organic solvents and harm to the human body. In recent years, there has been a growing demand for an aqueous coating composition to substitute for the organic solvent type coating composition.
A representative example of the aqueous coating composition includes an aqueous emulsion coating composition obtained by aqueous emulsion polymerization. As to a coating composition comprising a fluorine-containing copolymer as a main component, i.e., a fluorine-containing resin coating composition, many proposals have been made for an aqueous resin dispersion which can be used as an aqueous emulsion coating composition.
JP-A-2-225550 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes an aqueous resin dispersion obtained by emulsion polymerization of a fluoroolefin, a vinyl monomer having a polyoxyethylene group, and a hydroxyl group-containing monomer. JP-A-3-33148 proposes a process for the preparation of an aqueous dispersion of a fluorine-containing copolymer by emulsion polymerization of fluoroolefin monomers in the presence of a reactive emulsifying agent copolymerizable therewith.
In order to solve a problem common to aqueous coating compositions, i.e., poor water resistance of coating, the above proposals employ a process comprising an emulsion polymerization in the absence of an emulsifying agent by using the specific monomers or a process comprising copolymerizing an emulsifying agent upon polymerization.
As mentioned above, an aqueous fluororesin emulsion coating composition has heretofore been studied taking into consideration that the residual amount of low molecular weight emulsifying agent in the emulsion is minimized. However, the aqueous dispersion of fluorine-containing copolymer according to the prior art still exhibits poor film-forming properties and water resistance as compared with the organic solvent type fluororesin coating composition and thus has limited application.